1. Field of the Invention
This invention relates to a photochemical process for initiating the free radical reaction of hydrogen bromide with olefinic double bonds. More particularly, it relates to a photochemical process for initiating the hydrobromination of olefinic double bonds which involves a two-photon excitation of hydrogen bromide.
2. Description of the Prior Art
Free radical chain reactions are well-known processes which are of importance in the preparation of certain organic polymers from olefinic monomers, in the halogenation of alkanes, and in the hydrohalogenation of olefins. Typically, reactions of this type are initiated either by heating the reactants with an unstable radical precursor, such as a peroxide, or by photochemically generating appropriate free radicals in the presence of the reactants.
The free radical chain reaction of hydrogen bromide with olefins is a well-known process which can be initiated photochemically by irradiation of the reaction mixture with ultraviolet light having a wavelength shorter than about 300 nm. For the case of ethylene, this process can be described by the following equations: EQU HBr+h.nu..fwdarw.H.+Br. (1) EQU CH.sub.2 .dbd.CH.sub.2 +Br..fwdarw..CH.sub.2 CH.sub.2 Br (2) EQU .CH.sub.2 CH.sub.2 Br+HBr.fwdarw.CH.sub.3 CH.sub.2 Br+Br. (3) EQU Br.+.CH.sub.2 CH.sub.2 Br.fwdarw.CH.sub.2 BrCH.sub.2 Br (4)
Equation 1 sets forth an initiation reaction which involves the photochemical conversion of a molecule of hydrogen bromide to a hydrogen atom (H.) and a bromine atom (Br.). This photochemical conversion is caused by a photon (h.nu.) which has an energy in excess of the hydrogen bromide dissociation energy of 3.76 eV. Ethyl bromide is then produced by a free radical chain reaction, as set forth in Equations 2 and 3, which is initiated by the bromine atom formed according to equation 1. The bromine atom combines with an ethylene molecule to yield the .CH.sub.2 CH.sub.2 Br free radical (Equation 2) which, in turn, reacts with a molecule of hydrogen bromide to yield the ethyl bromide product and another bromine atom (Equation 3). The bromine atom produced according to Equation 3 then reacts with another molecule of ethylene according to Equation 2, and the propagation steps (Equations 2 and 3) continue to repeat until one of the free radical intermediates is destroyed in a termination reaction such as that illustrated by Equation 4.
The dissociation energy of hydrogen bromide is 3.76 eV. Accordingly, the initiation reaction (Equation 1) is carried out using photons having an energy which is equal to or in excess of this value. This consists of irradiating a reaction mixture of hydrogen bromide and an olefinic compound with light having a wavelength which is shorter than 330 nm (corresponding to an energy of 3.76 eV). In addition, the light must be of a wavelength which is absorbed by the hydrogen bromide. Since the light intensity from conventional lamp sources is extremely weak in the regions of the spectrum at which hydrogen bromide absorbs (i.e. at wavelengths shorter than 300 nm), the photochemical hydrobromination of olefins has been difficult to carry out efficiently using such sources. Alternatively, light having a wavelength shorter than 330 nm but longer than that at which hydrogen bromide absorbs can be utilized if the reaction mixture additionally contains a sensitizer such as acetone, benzophenone or acetophenone. This use of a sensitizer does not, however, involve a direct photolysis of hydrogen bromide. In addition, the use of sensitizers is not generally desirable since they result in the formation of undesirable byproducts.
U.S. Pat. No. 4,049,516, issued to Gellato et al. on Sept. 20, 1977, describes a conventional photochemical process for the hydrobromination of olefinic compounds. It is disclosed that hydrogen bromide absorbs light having a wavelength shorter than 300 nm and that the hydrobromination reaction can be initiated by direct photolysis of hydrogen bromide through the use of light having a wavelength which is not greater than 300 nm. In addition, it is disclosed that the hydrobromination reaction can be initiated indirectly with light of longer wavelength if a sensitizer such as benzophenone or acetophenone is incorporated into the reaction mixture. The photochemical addition of hydrogen bromide to 2-butene in the gas phase using a medium pressure mercury arc as a light source has been described in detail by G. A. Oldershaw et al., J. Chem. Phys., Vol. 41, Dec. 1, 1964, pp. 3639-3644.
The commercial preparation of ethyl bromide by the hydrobromination of ethylene has been carried out photochemically using high energy ionizing radiation such as gamma radiation from a cobalt-60 radiation source. This process has been described in Chemical Engineering Progress, Vol. 60, April 1964, pp. 33-36 and also in U.S. Pat. No. 3,145,155, which issued to Pumpelly et al. on Aug. 18, 1964. Although this process has been commercially successful, it requires the use of extensive shielding and the use of a hazardous radiation source. In addition, the use of high energy ionizing radiation such as gamma radiation is undesirable because each photon carries about one million times the amount of energy actually required to dissociate a molecule of hydrogen bromide into atoms. This excess energy is converted into heat and results in an increase in the temperature of the reaction mixture, which is undesirable since the quantum yield of the reaction decreases as the temperature increases.
Mercury arc lamps are conventionally utilized as light sources for photochemical applications. However, U.S. Pat. No. 4,417,964, issued to Wolfrum et al. on Nov. 29, 1983, describes the use of a laser as a light source in a photochemical process for the preparation of olefinic compounds by splitting off hydrogen halide from the corresponding saturated compound. Similarly, a laser has been utilized as a light source in various photochemical isotope separation processes. For example, U.S. Pat. No. 4,025,408, issued to Marling on May 24, 1977, discloses a process for deuterium enrichment which involves the use of laser-produced infrared photons to induce the addition of a hydrogen halide to an olefin. More specifically, a mixture of a hydrogen halide feedstock and an unsaturated aliphatic compound is irradiated to selectively vibrationally excite those molecules of the hydrogen halide containing the desired isotope of hydrogen to a predetermined vibrational level. The excited molecules of hydrogen halide then preferentially react with the unsaturated aliphatic compound to form a product which is enriched in the desired isotope. This is not, however, a free radical process.
Unstable noble gas halides such as XeF, XeCl, XeBr, KrF and ArF have found use as light emitting species in lasers since they can be easily formed in excited states by electron-beam pumping or discharge pumping of suitable gas mixtures. Such lasers are referred to as excimer lasers. For example, a mixture of 10% xenon, 89% argon and 1% fluorine can be pumped with 400 keV electrons to produce excited XeF which emits light of 351 nm wavelength. Similarly, ArF, KrF and XeCl can be utilized to generate light of 193 nm, 248 nm and 308 nm, respectively.
Ethyl bromide, which can be manufactured by the hydrobromination of ethylene with hydrogen bromide, is a commercially significant material which has found use as a refrigerant, as an ethylating agent in organic synthesis, and as a grain and fruit fumigant.